The present invention relates to a clad lid for electronic packages, in particular, to a clad lid with an identifiable clad lid.
Electronic packages are routinely sealed with covers. In U.S. Pat. No. 5,639,014 (Damiano et al.), Damiano et al. disclose a cover in its background of the invention. That cover comprises a substrate material, having a compatible coefficient of thermal expansion such as Alloy 42 or Kovar, plated with nickel as a corrosion barrier and gold as a solderable material. A gold and 20% tin eutectic solder is cladded to the gold plated layer of that cover. In most instances the solder is applied to just one side of the cover for economic reasons. A second gold plated layer is plated on the solder layer and the backing side (already a gold layer if solder is on one side).
Damiano et al. use a process to fabricate that cover that is disclosed in U.S. Pat. No. 4,737,418 (Slattery). The Slattery process comprises the steps of plating various layers of metal or alloys on a substrate and cladding a layer of metal or alloy on the top plated layer. The cladded substrate is then stamped to the desired cover size. Once the cover is stamped to the desired size, the cover is plated with a corrosion resistant metal, i.e., gold, to cover at least the ends of the cover and one side of the cover.
There are problems with the cover of Damiano et al. One problem is that a manufacturer must be able to distinguish the backing side from the solder side to properly orientate the cover on the electronic package. Damiano et al. recognize that it is extremely difficult to differentiate the two sides of its cover since both sides are colored gold. Damiano et al. propose two options to solve this problem. The first option is to form a solder window on the solder side and the other option is to apply solder to both sides. The latter option is extremely costly and inefficient. The solder window of the first option creates further problems.
There are two processes to fabricate the solder window. One process preforms the solder window and applies the preformed window onto the plated substrate. That process has numerous alignment problems that make it difficult to perform. The other process applies a mask material for the window and then applies the solder. The mask material can either be removed or not. In either process the second plated gold layer is applied to the solder layer and the sides. The resulting cover has a ridged solder side (if the mask material is removed or the preform process is used) and a smooth backing side.
The problems with the solder window are that (1) a proper seal between the cover and the electronic package may not occur and (2) the solder will not provide a solder junction that is visibly detectable. The first problem occurs when distinguishing the solder side from the backing side. The solder side is distinguishable from the backing side by feeling the ridge on the solder side. Feeling for the ridge can remove some of the solder and thereby destroy the desired seal between the electronic package and the cover.
The second problem occurs because a visible seal between the cover and the electronic package may not occur with a solder window. The thickness of the solder for clad lids is typically one third to half of that used in normal frame and lid assemblies. During the sealing of the lid, some of the molten solder from the interior of the lid is drawn to the sealing edge by capillary action. The sealing edge forms a solder junction considerably thicker than the original clad layer and provides a visual indication that the lid is sealed to the electronic package. If visual inspection is desired, then a solder junction by a solder window (with or without the mask layer) may not be acceptable.
The present invention solves the above-identified problems. The present invention alters the color of the solder side so that the backing side and the solder side are visually distinct from each other. Moreover, the solder side provides a solder junction that can be visually inspected.
A method of making an integral plated and solder clad cover lid for electronic packages is disclosed. The cover lid is plated and or clad with a first corrosion resistant and solderable material and is clad with a solder material on one side. The cladded substrate is stamped and then plated with a second corrosion resistant and solderable material. The second material is subsequently sintered and preferentially diffused into the solder material resulting in a visual distinction between the solder side of the cover lid and the backing side.